Many alkyl halides possess a degree of toxicity, sometimes very high toxicity. For example, the toxicity of methyl bromide is so great that it has been used for many years in the extermination of insects in mills, warehouses, vaults, ships, freight cars, and also as a soil fumigant for use by growers of strawberries, tomatoes, and other crops. Other applications include treatment of ships to remove rodents and treatment of foods such as fruits including dried fruits, grain, flour, nuts, and tobacco products to remove potential pests. Additionally, methyl bromide has been successful in fumigation against various microorganisms including fungi and bacteria. Recently, it has been advocated as the most effective agent against anthrax (Bacillus anthracis). Its virtues include the fact that it is not explosive, practically nonflammable, has very high diffusivity and permeability, and has been used safely for over 60 years.
Unfortunately, release of methyl bromide into the atmosphere is generally accepted to cause ozone layer depletion that can result in increased incidences of skin cancer. Thus, there is a need for methods of disposing of methyl bromide without releasing it to the atmosphere. In addition, there is a more general need for methods of rapidly and economically removing volatile alkyl halides such as methyl bromide from streams such as air or petroleum vapors.
Prior art methods for removal of alkyl halides can be divided into nondestructive and destructive ones. Most such methods in either category are not satisfactory for rapid removal of alkyl halides from gaseous streams and/or air volumes.
Non-Destructive Methods of Alkyl Halide Removal
Non-destructive methods of alkyl halide removal typically involve either scrubbing by dissolution of the halide in an appropriate solvent, or by adsorption onto a suitable adsorbent. Scrubbing solvents that work by dissolution of the alkyl halide include for example vegetable oil and marine oil (Canadian Patent 1,282,317). Examples of applicable adsorbing agents include zeolites (U.S. Pat. No. 4,309,281), and activated carbon (J. G. Leesch, G. Knapp, B. E. Mackey, Methyl Bromide Adsorption on Activated Carbon to Control Emissions from Commodity Fumigations, also: J. G. Leesch, G. Knapp, B. E. Mackey, J. Stored Prod. Res., 36, 65, 2000). Methods based on the use of adsorbents or scrubbing solvents suffer from non-discriminatory scrubbing of other volatile compounds in the gas streams, resulting in the formation of unwanted degradation products during regeneration of the adsorbent or scrubbing solvent.
Destructive Methods of Alkyl Halide Removal
Japanese Kokai JP 49-127,862 discloses a method involving a reaction of methyl bromide in isopropanol with ethanolamine dissolved in water. The method takes advantage of a nucleophilic substitution reaction that is, however, not sufficiently rapid for a variety of applications. Another method employs bioreactors and specific species of a-Proteobacteria that can directly oxidize and grow on methyl halides (L. G. Miller, S. M. Baesman, R. S. Oremland, Use of Bioreactors to Remove Methyl Bromide Following Contained Fumigations, Proceedings of 2002 Annual International Research Conference on Methyl Bromide Alternatives and Emissions Reductions; also: L. G. Miller, S. M. Baesman, R. S. Oremland, Bioreactors for Removing Methyl Bromide following Contained Fumigations, Environ. Sci. Technol., 37, 1698, 2003). Yet another destructive method (F. G. Belmonte, K. J. Abrams, J. P. Oppenheim, U.S. Pat. No. 6,207,120 B1; Mar. 27, 2001) proposes to heat and mix a vent gas containing an alkyl halide with a combustible fluid, followed by catalytic oxidation of the mixture. In U.S. Pat. No. 5,904,909, Yates discloses a method of destroying methyl bromide, for example from a fumigation gas stream, by adsorption or retention of that compound on a substrate such as carbon, followed by destruction of the adsorbed methyl bromide by treating it with thiosulfate. They state “if methyl bromide gas is merely bubbled through an aqueous solution of thiosulfate, decomposition of methyl bromide is very slow and incomplete (about 20-30% decomposition after 2 hours).”
Despite these advances, there continues to be a need for rapid and efficient means for removing alkyl halides at low and moderate concentrations from gas streams.